1. Field of the Invention
The present invention relates to a laminated ceramic electronic component and a production method therefor, and to an electronic device having such a laminated ceramic electronic component. More particularly, the present invention relates to an improvement in the size of via-hole conductors formed in a laminated ceramic electronic component.
2. Description of the Related Art
A laminated ceramic electronic component, which is related to the present invention, is also called a “multilayer ceramic substrate”, and includes a laminated member having a laminated structure including a plurality of ceramic layers.
Wiring conductors are provided for specific ceramic layers disposed in the laminated member, thereby constituting a desired circuit. Wiring conductors include, for example, a via-hole conductor extending through the specific ceramic layers or a conductor film extending along the principal surface of the ceramic layers. The conductor film includes an inner conductor film disposed inside the laminated member and an outer conductor film disposed on the outer surface of the laminated member.
Passive elements, such as capacitors, inductors, triplate structures, and/or microstrip lines, are sometimes built in the laminated member. A part of the via-hole conductor and a part of the inner conductor film are used to constitute such passive elements. Active elements, such as semiconductor IC chips, and, as necessary, some of the passive elements, are sometimes mounted outside the laminated member. Part of the above-described outer conductor film functions as a terminal for electrically connecting such mounted elements.
The laminated ceramic electronic component combined as described above is mounted on an appropriate wiring board so as to constitute a desired electronic device. When the laminated ceramic electronic component is mounted on the wiring board, a part of the above-described outer conductor film functions as a terminal for electrically connecting the laminated ceramic electronic component to the wiring board.
For example, such a laminated ceramic electronic component is used as an LCR high-frequency composite component in the field of mobile communication terminals, and is used as a hybrid component, which is a combination of an active element, such as a semiconductor IC chip, and a passive element, such as a capacitor, an inductor, and a resistor, or is simply used as a semiconductor IC package in the computer field.
More specifically, the laminated ceramic electronic component is widely used to constitute various electronic components, such as PA module boards, RF diode switches, filters, chip antennas, package components, and hybrid devices.
In order to meet the growing demand for higher frequency, the ceramic layers in such a laminated ceramic electronic component are made of dielectric materials having low dielectric constants in most cases. Furthermore, in order to facilitate co-firing for obtaining a laminated member, it is preferable that the ceramic layers be made of ceramics having the same dielectric constant, that is, having the same composition.
With such a background, when a passive element is built in the laminated member, as described above, the thickness of the ceramic layer is changed in accordance with the type of the passive element. This will be described with reference to FIGS. 4 to 6.
FIG. 4 is a cross-sectional view illustrating a capacitor 1 defining a built-in element. The capacitor 1 includes a plurality of capacitor electrodes 3 defined by inner conductor films and opposing one another with ceramic layers 2 therebetween. In order for such a capacitor 1 to be compact and to have a large capacitance, thicknesses T1 and T2 of the ceramic layers 2 are decreased.
FIG. 5 is a cross-sectional view illustrating a triplate structure 4 defining a built-in element. The triplate structure 4 includes a center conductor 5 defined by an inner conductor film, and a pair of ground conductors 7 and 8 similarly defined by inner conductor films and placed on both sides of the center conductor 5 with ceramic layers 6 therebetween. In such a triplate structure 4, the thicknesses of the ceramic layers 6 are increased in order to increase a distance S between the ground conductors 7 and 8.
FIG. 6 is a cross-sectional view illustrating two capacitors 9 and 10 defining built-in elements. In order to prevent coupling between the electrostatic capacitances of the two capacitors 9 and 10, a thickness T of a ceramic layer 11 interposed between the capacitors 9 and 10 is increased.
In such a case in which a built-in element is placed inside the laminated member, the optimal thickness of the ceramic layer positioned in relation to the built-in element varies depending on the type of the built-in element. For this reason, it is necessary to mix a plurality of types of ceramic layers of different thicknesses in the laminated member.
On the other hand, a plurality of via-hole conductors extending through a specific ceramic layer in the laminated member generally have the same sectional size. This is because efficiency of manufacturing is decreased when a plurality of via-hole conductors of different sectional sizes are formed.
For example, a method shown in FIG. 7 is adopted to form via-hole conductors.
Referring to FIG. 7, through holes 13 are formed in a ceramic green sheet 12, which will become a ceramic layer, in a laminated member of a laminated ceramic electronic component. The ceramic green sheet 12 is placed on a suction device 14.
The suction device 14 has a vacuum chamber 15. Negative pressure is applied in the vacuum chamber 15, as shown by arrow 16. The opening of the vacuum chamber 15 is closed by a suction plate 17 having multiple minute air paths (not shown).
A porous sheet 18 formed of paper or another filter material is placed on the upper surface of the suction plate 17. The ceramic green sheet 12 is arranged to contact with the porous sheet 18.
When negative pressure is applied in the vacuum chamber 15 in such a state, as shown by the arrow 16, it is also exerted on the through holes 13 via the suction plate 17 and the porous sheet 18.
In this state, a conductive paste 19 is filled into the through holes 13 by using screen printing. That is, the conductive paste 19 is applied on a screen 20, is moved with the movement of a squeegee 21 along the screen 20, and is filled in the through holes 13 during the movement by the action of the above-described negative pressure. The conductive paste 19 in the through holes 13 forms via-hole conductors 22.
Next, the ceramic green sheet 12 is peeled off the porous sheet 18. Before or after peeling, the conductive paste 19 for the via-hole conductors 22 is dried.
In the above-described peeling step, however, a part of the conductive paste 19, which is filled in the through holes 13 so as to form the via-hole conductors 22, adheres to the porous sheet 18, as shown in FIG. 8, and therefore, the amount of the conductive paste 19 in the through holes 13 is insufficient. Such insufficient filling resulting from the loss of the conductive paste 19 after filling causes defective continuity between the via-hole conductor 22 and another via-hole conductor or another wiring conductor such as a conductor film.
A method shown in FIG. 9 may be used to form the via-hole conductors 22. In FIG. 9, the components corresponding to the components shown in FIG. 7 are denoted by like numerals, and repetitive description thereof is omitted.
Referring to FIG. 9, a ceramic green sheet 12 is handled while being backed with a carrier film 23. Through holes 13 are formed through the ceramic green sheet 12 and the carrier film 23.
In a manner similar to that shown in FIG. 7, a porous sheet 18 is placed on the upper surface of a suction plate 17 of a suction device 14. The ceramic green sheet 12 backed with the carrier film 23 is arranged in contact with the porous sheet 18 so that the carrier film 23 is positioned on the upper side of the ceramic green sheet 12.
In such a state, negative pressure is applied in a vacuum chamber 15, as shown by arrow 16. The negative pressure is also exerted into the through holes 13 via the suction plate 17 and the porous sheet 18, and a conductive paste 19 is applied on the upper surface of the carrier film 23. The conductive paste 19 is moved with the movement of a squeegee 24 along the upper surface of the carrier film 23, and is filled into the through holes 13 during the movement by the action of the above-described negative pressure.
By thus filling the conductive paste 19 in the through holes 13 from the side of the carrier film 23 which functions as a mask without using the screen 20 shown in FIG. 7, via-hole conductors 22 are formed in the through holes 13.
In a case in which the above method shown in FIG. 9 is adopted, a conductive paste film functioning as a conductor film is formed by printing a conductive paste on the principal surface of the ceramic green sheet 12 which faces outward, while the ceramic green sheet 12 is backed with the carrier film 23.
The ceramic green sheet 12 is thus handled while being backed with the carrier film 23 because it is weak and is quite difficult to handle alone. By handling the ceramic green sheet 12 while being backed with the carrier film 23, it is possible to facilitate handling and positioning of the ceramic green sheet 12 in each step. Moreover, it is possible to reduce variations in degrees of shrinkage of the ceramic green sheet 12 when the conductive paste 19 for forming the via-hole conductors 22 and the conductive paste film is dried.
When obtaining a laminated member for a desired laminated ceramic electronic component, a plurality of ceramic green sheets including the ceramic green sheet 12 are stacked. Before stacking, the carrier film 23 must be peeled off the ceramic green sheet 12. In this case, a part of the conductive paste 19 filled in the through hole 13 is sometimes removed together with the carrier film 23, and the amount of the conductive paste 19 in the through holes 13 becomes insufficient.
In the method shown in FIG. 9, of course, when the ceramic green sheet 12 is peeled off the porous sheet 18, a part of the conductive paste 19 filled in the through holes 13 is also sometimes removed and adheres to the porous sheet 18, as shown in FIG. 8.
Insufficient filling of the conductive paste 19 in the through hole 13 occurs not only due to the causes described with reference to FIG. 8 or FIG. 10, but also in the following cases.
For example, in a case in which the ceramic green sheet 12 is relatively thin, the shape-maintaining strength of the conductive paste 19 filled in the through holes 13 is relatively low. For this reason, at least a part of the conductive paste 19 sometimes falls out when handling the ceramic green sheet 12.
After the via-hole conductors 22 are formed, a conductive paste film functioning as a conductor film is sometimes formed on the ceramic green sheet 12 by screen printing. In this case, an emulsion film is formed on a surface of a screen used in screen printing, which surface opposes the ceramic green sheet 12. When the screen is pressed by a squeegee, the emulsion film moves into contact with the ceramic green sheet 12 and then moves apart therefrom along the shape of the leading end of the squeegee. As the result of such a movement of the emulsion film, a part of the conductive paste 19 filled in the through holes 13 sometimes adheres to the emulsion film and is removed to the side of the screen.
In order to form dense via-hole conductors 22 with high electrical conductivity, the conductive paste 19 used to form the via-hole conductors 22 has a higher content of metal components than that of the conductive paste for a conductor film. For this reason, the amount of resin components contained in the conductive paste 19 is relatively small, and as a result, the shape-maintaining strength of the conductive paste 19 is decreased. Because of this, the above-described insufficient filling of the conductive paste 19 in the through holes 13 is more likely to occur due to the loss after filling.
As described above, insufficient filling is more likely to occur due to the removal of a part of the conductive paste 19 filled in the through holes 13 as the sectional size of the through holes 13 increases and the height thereof decreases. Therefore, it is conceivable to decrease the sectional size of the through holes 13 and to increase the height thereof in order to prevent the conductive paste 19 filled in the through holes 13 from being removed. However, such measures to decrease the sectional size of the through holes 13 and to increase the height thereof cannot be adopted simply.
That is, for example, in a case in which the conductive paste 19 is applied by screen printing, as shown in FIG. 7, when the sectional size of the through holes 13 is small, high accuracy is required to align the screen 20 and the ceramic green sheet 12, and the conductive paste 13 cannot easily enter the through holes 13. For this reason, insufficient filling of the conductive paste 19 occurs, and a long time is needed to fill the conductive paste 19. These problems are pronounced, in particular, when the thickness of the ceramic green sheet 12 is large, that is, when the height of the through holes 13 is large.